Today's 'Extra' Full Moon is Last of 2012

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Today's final full moon of 2012 is a somewhat rare occasion: It's
the 13th full moon of the year.

That's a little odd, given that there are 12 months in a year,
and months are based on the lunar cycle. But extra
full moons happen periodically, because the moon's phases
don't line up perfectly with our calendar. In fact, the moon
takes just over 29.5 days to go from full to new to full again.
Do the math, and it turns out there are actually 12.37 lunar
cycles each year — just a smidge over an even 12.

That means that occasionally, you get two full moons in one
month, an occurrence known colloquially as a "blue moon." It
happened this year in August and won't happen again until
2015. Before 2012, the last blue moon occurred on Dec. 31, 2009.

The moon was at its fullest point this morning at 5:21 a.m.
Eastern Standard Time (10:21 UTC), but will appear completely
round to the naked eye through the day and night. Native American
tribes in the eastern United States traditionally referred to
December's full moon as the Full Cold Moon or the Long Nights
Moon because of the frigid weather and winter solstice that
brings the longest darkness of the year. [ Gallery:
Fantastic Full Moons ]

As the moon waxes and wanes, you might wonder why we don't just
see its fullness all the time. The reason has to do with how the
moon, sun and Earth align. When the moon's rotation brings it in
line with Earth on the far side from the sun, the sun's rays hit
the Earth-facing side of the moon directly, illuminating it and
causing a full moon. When the moon rotates between the Earth and
the sun, the sun's glare washes it out, creating an invisible new
moon.

One half of the moon is always illuminated by the sun, but that
doesn't mean we always see that half. When the alignment is such
that the view from Earth shows both the lit-up and shadowed sides
of the moon, we see those in-between moon shapes known as
crescent, half and gibbous (more than half illuminated, but not
full). A waxing moon is on its way to becoming a full moon, while
a waning one is headed toward new moon territory.

While a blue moon is a neat occurrence, a few years are even more
special. It's possible for some years to contain not one, but two
blue
moons. The last time a double blue moon year occurred was in
1999. It'll happen again in 2018.

Editor's Note: This article was corrected at
2:23 p.m. Eastern Time to fix an error in the maximum number of
full moons per year.